[123.09] Further Evidence for a Merger Origin for the Thick Disk: Galactic Stars Along Lines-of-sight to Dwarf Spheroidal Galaxies

The history of the Milky Way Galaxy is written in the
properties of its stellar populations. Here we analyse stars
observed as part of surveys of local dwarf spheroidal
galaxies, but which from their kinematics are highly
probable to be non-members. The selection function --
designed to target metal-poor giants in the dwarf galaxies,
at distances of around 100 kpc -- includes F-M dwarfs in the
Milky Way, at distances of up to several kpc. The stars
whose motions are analysed here lie close to the cardinal
directions of Galactic longitude \ell=270circ and
\ell=90circ, where the radial velocity is sensitive to
the orbital rotational velocity, V\phi. We demonstrate
that the faint F/G stars contain a significant population
with V\phi ~100 km/s, which does not match either the
canonical thick disk or the stellar halo. We argue that this
apparently pervasive population has the expected properties
of `satellite debris' in the thick-disk/halo interface,
which we interpret as remnants of the merger that heated a
pre-existing thin disk to form the thick disk.